Astronomers may have detected the first instance of a “superkilonova” – a rare, powerful explosion resulting from the merger of two neutron stars, potentially including one smaller than previously thought possible. The event, designated AT2025ulz and observed 1.3 billion light-years away, initially appeared as a textbook kilonova before exhibiting unusual behavior that blurred the lines with a standard supernova.
The Nature of Kilonovae and Supernovae
Kilonovae are among the most violent events in the cosmos, forged when two neutron stars collide. Unlike supernovae (the typical death throes of massive stars), kilonovae synthesize heavy elements like uranium and gold, dispersing them across the universe. The first confirmed kilonova, GW170817, was observed in 2017 alongside gravitational waves, providing direct evidence of neutron star mergers. However, these events remain poorly understood, and new observations continue to push the boundaries of our knowledge.
The Peculiar Case of AT2025ulz
On August 18, 2025, the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo detected gravitational waves signaling a neutron star collision. A team at the Palomar Observatory quickly identified a fading red source matching the expected signature of a kilonova. But three days later, the object unexpectedly brightened again, shifting to blue wavelengths characteristic of a supernova – confusing astronomers.
The Sub-Solar Neutron Star Hypothesis
The key anomaly lies in the gravitational wave data, which suggests at least one of the colliding neutron stars may have been smaller than the sun ; neutron stars typically range from 1.2 to 3x the sun’s mass. This raises fundamental questions about how such a small neutron star could form. Researchers propose two scenarios:
- A rapidly spinning star explodes as a supernova before fully fissioning into two sub-solar neutron stars.
- A collapsing star forms a disk of debris that eventually coalesces into a smaller neutron star, akin to planet formation.
The team hypothesizes that these “forbidden” neutron stars could then merge, triggering a kilonova within the expanding supernova, explaining the initial red wavelengths overtaken by the supernova’s blue glow.
Why This Matters
This potential superkilonova is significant because it challenges existing models of neutron star formation and merger events. If confirmed, it suggests that smaller neutron stars can exist and collide, broadening our understanding of how heavy elements are produced in the universe. The study’s lead author, Mansi Kasliwal, notes that even “failed” candidates like AT2025ulz are valuable:
“Everybody was intensely trying to observe and analyze it, but then it started to look more like a supernova, and some astronomers lost interest. Not us.”
Continued research into these ambiguous events is crucial, as they hold the key to refining our understanding of the most extreme phenomena in the cosmos.
The discovery underscores the need for persistent observation and analysis, even when initial data appears inconclusive. The universe rarely yields its secrets easily, and unexpected anomalies often lead to the most profound breakthroughs.
